A highly popular, non-toxic water-based acrylic wash produced by Games Workshop for use on plastic, metal, and resin miniatures. specifically designed to flow into the recesses of models to create automatic depth, shadows, and definition.
Ideal for shading metals (like gunmetal and silver), deep folds, mechanical parts, and cold color palettes. 
Use a larger brush with a fine tip rather than a tiny brush. This helps draw the fluid smoothly across surfaces and prevents "coffee staining" (ugly rings left behind when excessive wash pools and dries unevenly on flat areas).
If the wash settles heavily in deep crevices, it can occasionally dry with a slight sheen. You can easily correct this by applying a standard matte varnish over the completed miniature to unify the final look.
While it can be used straight out of the pot, diluting it slightly with a touch of water or acrylic medium on a palette helps prevent it from staining or darkening your lighter base coats too aggressively.